Typically, in order to function properly, chip circuitry requires a generally stable reference voltage, such as a band gap reference voltage. Thus, whether the chip is operating in either standby mode or in active mode, the output band gap reference voltage should remain generally stable. For instance, in a memory device, the band gap reference voltage should remain relatively stable both during the absence of line activity and during functions such as read, program, or verify.
Reference is now made to FIGS. 1A, 1B, and 1C which illustrate prior art circuits which supply band gap reference voltages.
FIG. 1A illustrates a circuit 10 comprising a band gap reference generator 12 and a buffer 14. Generator 12 supplies a band gap voltage, generally referenced by arrow 16, to buffer 14, which drives the voltage and supplies an output band gap reference voltage, generally referenced by arrow 16', to an output 18. Buffer 14 is continuously on, thereby effectively eliminating noise in the output line 16 as generally experienced during typical transients in the load circuit.
Generator 12 is a low power, high resistance driver which consumes relatively little energy. However, it reacts slowly to voltage changes in the line, and thus takes a relatively long time to stabilize after initialization of the line activities such as read, program, or verify. In order to compensate for the slow reaction time of the generator 12, buffer 14 drives the current, thereby improving response time. However, the decrease in reaction time is counterbalanced by an increase in the constant energy consumed by circuit 10.
FIG. 1B illustrates another circuit 20 comprising a band gap reference generator 22, which supplies a band gap reference voltage, generally referenced 26, directly to an output 28. The voltage level of reference voltage 26 is generally equivalent to that of reference voltage 16'.
Generator 22 is a high power, low resistance driver which while consuming more energy than generator 12, reacts more quickly and stabilizes faster after changes in the line. Since generator 22 reacts quickly, circuit 20 eliminates the need for a buffer 14, while still supplying generally the same reference voltage as that supplied by circuit 10. Nevertheless, circuit 20 is a continuous high power consumer.
FIG. 1C is a prior art circuit taught in U.S. Pat. No. 5,568,085 to Eitan et al, and is included herein by reference. FIG. 1C illustrates circuit 30 comprising both low power generator 12, high power generator 22, and a NMOS switching transistor 32. Circuit 30 supplies a low power band gap voltage, generally referenced by an arrow 36, to an output 38. Alternatively, circuit 30 also supplies a high power band gap voltage, generally referenced by an arrow 36'.
The drain of transistor 32 is connected to high power generator 22 while the source of transistor 32 is connected to a ground source V.sub.SS. The gate of transistor 32 is connected to a controlling line 34, which supplies a control signal V.sub.CS. Control signal Vcs controls whether the high power generator 22 is off or providing high power band gap voltage 36' to output 38.
Low power generator 12 is always on, while high power generator 22, when so indicated, overrides the low power generator 12. The high power generator 22 is controlled by transistor 32 which is controlled by control signal V.sub.CS.
When V.sub.CS is high, transistor 32 is activated, and therefore high power generator 22 is on, and supplies high power band gap voltage 36'. And when V.sub.CS is low, transistor 32 is deactivated, thus high power generator 22 is off, and only low power generator 22 supplies low power band gap voltage 36. In contrast to circuits 10 and 20 which are continuous high power consumers, circuit 30 from time to time consumes high power; only when the high power generator 22 is on. However, since there are two independent generators operating in circuit 30, there is an inherent mismatch in the band voltages 36 and 36'.